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Old 31st May 2020, 9:17 pm   #1
Wendymott
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Default Faraday chamber.

Hi Peeps.. Another "lock down" project....While researching my latest project "Triband VHF Transceiver"....Working down in the RF Microvolts area, I have a lot of QRM from local FM broadcast transmitters Band 2.
I decided that I should try and make a Faraday chamber . As I didnt have any TIN chokky / sweetie containers.... all plastic now...I found an old "Aluminium in ? brackets" flight case..... Ideal size... just waiting for re purposing.
However the construction of these is rather poor, 5mm of Hardboard. 1mm of aluminium coating...hardly keep much RF out.
Fortunately I also have some large sheets of Copper clad plastic "of sorts", so I made an inner shell... See photo's
BNC and SMB sockets allow interface with the outside world... but what do I use for DC.
Should I use filters that are in the Band 2 area, or should I look for a wider rejection. The DC levels will be +12V 100 m/A max
Also the audio output, again.. like the DC port, or just use a BNC connector
Your suggestions would be appreciated
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Old 31st May 2020, 10:23 pm   #2
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Default Re: Faraday chamber.

In the mid 1970's I was system testing a UHF (400 - 500 MHz band) data link we were developing for a third party. To do the link test the transmitter was mounted in a large die cast Eddystone box and connected to the receiver by ordinary 50 Ohm cable (Uniradio 43 from RS) via a switched attenuator. We did originally use BNC connectors but had problems with erratic and unrepeatable results.

To cut a long story short, the problem was eventually traced to the BNC connectors, established by replacing the connectors with type N and SMA, and making short N to BNC cables for the switched attenuator, and soldering up the attenuator's BNC connectors solid with plenty of solder. The measurement problems then vanished and we coud get over 120dB of isolation, even with the receiver resting on the transmitter box.

I can't recall what we used for DC and signal connections. I have a feeling it was plain Oxley barb unfiltered feedthroughs, but it could have been filtered feedthroughs. It was either the Oxley PTFE insulated push-in, or else a screw-type fixing, because we couldn't solder to the Mazac metal of tte Eddystone box. We didn't need RF gasket for the box lid, and didn't even need to screw the lid down to get full isolation, although this was always done when making measurements.

The main point was that , although we were using top quality BNC connectors (Suhner, Radiall, Greenpar, Amphenol), they all had too much leakage. Another division of Plessey, from who we got the idea of using a large Eddystone box for this type of thing, was I think using miniature push- on connectors ( either SMB or SMC, I forget which ). They had attenuators with these newer connector types, whereas our attenuators all had BNC, and the connector type was the only difference between our respective set-ups.

The client was very pleased that this arrangement worked so well. They had budgetted for building a special screened room, with expensive triax cable etc. for testing, whereas our arrangement meant the test could be done on an ordinary lab bench.

Last edited by emeritus; 31st May 2020 at 10:42 pm. Reason: Typos
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Old 31st May 2020, 10:55 pm   #3
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Default Re: Faraday chamber.

Um, that isn't actually a Faraday chamber, you know?

If it was one, it would probably be useless for what you want it for. Faraday shielding is very special. It stops only electric fields and lets magnetic fields go straight through.

Faraday invented the screen named after him as a means to demonstrate that his newly invented transformer operated by magnetic coupling, not capacitive.

Unfortunately the prefix 'Faraday' gets added whenever a lot of people really mean just an ordinary screen, one which tries to stop everything. It's as if every time the word 'Tyre' was used, people automatically wrote 'Flat Tyre'

Pet hate, but it's not just wrong it's embarrassingly the opposite of what's usually meant.

David
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Old 31st May 2020, 11:27 pm   #4
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Default Re: Faraday chamber.

Neat project! Over the years I've had several attempts to make something similar, but usually much smaller! I didn't have a lot of success unless the box had a decent EMC gasket around the lid faces. Even this didn't work as well as I'd hoped. In the end I bought a used shielded enclosure quite cheaply and it has been brilliant. It uses feedthrough caps for the DC connections although it also has a 9 way D type interface that has feedthroughs built into the connector. It also has several SMA RF connectors.

The feedthrough caps on the dc power feed connections work really well. For comparison, if I try and trap a really skinny wire in via the door seal then this completely compromises the performance.

I haven't tested it properly since I bought it but back then I could put a 32dB gain dual MMIC amp inside the box with a pickup antenna with the output fed to a spectrum analyser. When tested for VHF FM stations or for mobile phone signals the shielding results were amazing as the door was shut and the pressure lock was clicked into place. The signals simply vanished. However, If I repeated the test with a dc power wire trapped in the door rather than being fed in via the feedthroughs the chamber hardly did anything at all. The dc power wire trapped in the door acted as a kind of repeater antenna into the box even though the EMC mesh seal around the door was well squished shut.

The reason I'm telling you all this is that I think you will need to fit some sort of compression EMC seal around the lid edges and you need to use proper feedthrough caps on the dc connections. Otherwise the box may achieve very little in the way of shielding. It might also be worth lining the insides with ferrite and RF absorber. That is what is inside my commercial version although I'm not sure how much this helps.
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Old 31st May 2020, 11:35 pm   #5
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Default Re: Faraday chamber.

Faraday shield- the brownish rectangle in the picture is copper mesh bisecting the aerial input transformers, allowing magnetic field coupling but not electric field coupling between primary and secondary windings. (The receiver in question was under rebuild at the time, hence all the gaps and disconnects!)
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Old 1st Jun 2020, 12:17 am   #6
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Default Re: Faraday chamber.

I just retested mine with a 60cm long pickup wire inside the shielded enclosure. This antenna is then fed via coax to one of the SMA bulkhead connectors and this then goes to a spectrum analyser via another coax cable. The analyser has an internal preamp giving it a typical noise figure of just over 4dB. The screenshot below shows that nothing from a VHF FM station gets inside the shielded box to reach the pickup antenna.

The same antenna shows FM signals up to about -70dBm when moved outside the enclosure.

If you fit a decent (squishy braided) EMC seal around the lid interface and use feedthroughs for the dc connections I think you could get similar performance. It is just a metal box but the seal on the door or lid has to be good! You might be able to ask nicely at one of the UK EMC gasket manufacturers to see if they have any EMC seals that are end of reel or old stock. Eg TC Shielding in Ross on Wye might have something. Tell them you are a radio ham and send a pic of your enclosure. They might be able to send you some free samples?
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Old 1st Jun 2020, 12:31 am   #7
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Default Re: Faraday chamber.

Quote:
Originally Posted by Radio Wrangler View Post
Um, that isn't actually a Faraday chamber, you know?

If it was one, it would probably be useless for what you want it for. Faraday shielding is very special. It stops only electric fields and lets magnetic fields go straight through.

Faraday invented the screen named after him as a means to demonstrate that his newly invented transformer operated by magnetic coupling, not capacitive.

David
Hi David,

I'm curious and have been searching on the internet for details of what (exactly) Faraday's original shield was and can't find anything. I'm especially interested in exactly how his screen differs from what is apparently the common misconception of a wire mesh, steel box or even an aluminum flight case. Can you point us in the right direction? I thought I had it straight in my mind but after reading a few recent posts on the subject I realise I might not actually understand as much as I thought I did, especially after reading some of your own condemnations of the phrase "Fraday cage" which (in the context they were used) did not seem to me to be too troublesome. All this has left me feeling that there's a subtlety which you are more than aware of and, unfortunately I am not!

Thanks,
Steve.
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Old 1st Jun 2020, 8:03 am   #8
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Default Re: Faraday chamber.

Faraday had faced accusations that the coupling in his transformer was nothing more than capacitance, so he devised an electrostatic screen to show it made no difference.

A lot of people have real Faraday screens actually doing the very thing he devised it for. Plenty of mains transformers have a 'screen' connection. This goes in to connect to a layer of foil between the mains and secondary windings. The foil does NOT form a completed turn as a circuit. That would take current and disrupt the magnetic coupling as well a sthe electric coupling. What it does do is interrupt the capacitive path from primary to secondary. THe primary has capacitance to the screen, and the screen has capacitance to the secondary. But because the screen is grounded with a low impedance connection, the current in the primary-screen capacitance is diverted to ground, leaving a lot less voltage on the screen to drive the screen to secondary capacitance.

These screens work well on transients on the mains and reduce 'fridge clicks'

Oddly, the interwinding screen doesn't usually get the Faraday- prefix.

Meshes act as screens where the mesh nature allows circulating eddy currents to form.
A grille, with wires in only one direction ( best only connected together in one place per wire) doesn't allow eddy currents to circulate so mag fields are unperturbed, but electric fields are blocked.

Magnetic loop antennae are another thing often provided with a Faraday screen. They wouldn't be much use with a fully effective screen, and their Faraday screen acts to protect them from near-field electric field interference.

So, like in Turretslug's photo, there are plenty of applications where we want magnetic coupling but not electric coupling.

Like in Wendy's attache case, there are plenty of applications where we want to shield against everything.

Sometimes you want a saucepan, sometimes you want a colander. Keeping the names unconfused avoids confusion

David
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Old 1st Jun 2020, 8:21 am   #9
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Default Re: Faraday chamber.

I should have mentioned that the very fine copper mesh illustrated in my previous post has bewilderingly fine silk insulation on the wires, they're only connected together at the metal earthing bracket at the top of the picture- as David says, this is fundamental to the effective operation of the screen.

Quite some time ago, there was a picture on the forum of a copper bar Faraday screen between primary and secondary of the aerial coupling in a slightly intimidating '30s 500kW MF transmitter,

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Old 1st Jun 2020, 9:56 am   #10
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Default Re: Faraday chamber.

I really have limited experience of this stuff but it might be enough to just have a few contact points around the lid face. Ideally you want something that lets you open and close the box easily.

There are various options for this. At work we have a good relationship with TC Shielding and they do things like (metal) fabric over sponge and springy beryllium 'fingerstrip' contacts. See the links below.

https://www.tcshielding.com/fingerstrips.htm

Quote:
The robust nature of the product makes it ideal for enclosures which require a high frequency of access such as cabinets, cases and doors.
The individual springy fingerstrips can be quite big (over 3cm long and 1cm wide) and it might be possible to fit several of them such that they sit sideways and get squished inwards as the lid faces meet.

The metal fabric over sponge strip works quite well too and you probably won't need a continuous strip of it.

https://www.tcshielding.com/fabwrap.htm

It comes in all shapes and profiles and you might only need a couple of feet of it cut into short strips.

So this could be a simple sample request from TC.
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Old 1st Jun 2020, 11:30 am   #11
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Default Re: Faraday chamber.

Thanks David - I see it clearly now

Steve.
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Old 1st Jun 2020, 12:34 pm   #12
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Default Re: Faraday chamber.

Hi Jeremy David Colin and other commentators.
Ok I may have got the name wrong..not important...
What i needed and have got is the answer to my question....Thanks Jeremy ...... it is not intended to be anything except a Local FM station block. Jeremy I have some nice copper braid, of which I made the lid to case connectors, from some RG whatever coax.... I was looking at a similar thought yesterday which I will now implement.
I will use some 1 nf Feed through caps for the DC and Audio out, as I have a fair few.
I will report back when I have tested it, with and without a lid seal..but thanks guys..
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Old 1st Jun 2020, 1:04 pm   #13
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Default Re: Faraday chamber.

Feed through capacitors are great, just where you pass power/audioo through the copper surface, but their effect is multiplied by having a little toroid with several turns on it.... on each side of the feedthrough.

Professional EMC chamber door end up in a metal edge folded perpendicular to the plane of the door, along all four sides which plugs into a channel in the door frame. THe channel is stuffed with beryllium copper springy finger strips that contact the edge of the door on both of its faces. Like a T plugging into a U in cross section and the inside of the U having the finger stock inserted.

David
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Old 1st Jun 2020, 1:39 pm   #14
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Default Re: Faraday chamber.

Yes, braid can work really well. My commercial unit uses two strips of braid inside the cover/door. Like your box it has inner and outer walls and as the door closes there are two faces that bite into braid as per the attached image below. This hopefully gives a really tight RF seal.

However, as I've said before, if I allow a tiny skinny wire like a fine insulated strand of alarm wire to be jammed inside this door (as a dc power lead for example) then the performance of the shielded box really does suffer. I think the wire can act as a repeater antenna or an efficient transmission line even though it has been squished mercilessly into the door seal.
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Old 1st Jun 2020, 9:22 pm   #15
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Default Re: Faraday chamber.

Hi David and Jeremy...... All noted..... The first photo's didnt show the lid being incomplete..I was not going to invest more copper on the sides of the lid until I had a sense that it was going to be of value... after initial testing I decided to add the missing copper. I also fitted copper foil in the corners of the main box and a strip of copper along part of the top edge to act as a positive contact. The two analyser photos show the desired end result... two thing I did note was.. it is important to keep the lead from the analyser to the box as short as possible...otherwise I get Band 2 interference... and the other is that I really need to increase the number of bolts securing the outer skin to the inner copper, there is a hand capacity effect...so I have a lot of drilling to do tomorrow..but it was a worthwhile investment in time and materials
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Old 1st Jun 2020, 10:26 pm   #16
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Default Re: Faraday chamber.

It should prove to be a very useful piece of equipment and well worth the effort to make it. I use mine mainly for noise figure testing but it is also nice to use for spurious free dynamic range testing especially if the circuit under test has an exposed RF filter.

If you make yourself a nice RF noise source you can use the shielded box and that Siglent spectrum analyser to do noise figure testing of RF amplifier or RF mixer stages. Without the shielded box this would be quite difficult to do.
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Old 2nd Jun 2020, 12:24 am   #17
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Default Re: Faraday chamber.

Quote:
Originally Posted by Radio Wrangler View Post
metal edge folded perpendicular to the plane of the door, along all four sides which plugs into a channel in the door frame. THe channel is stuffed with beryllium copper springy finger strips that contact the edge of the door on both of its faces. Like a T plugging into a U in cross section and the inside of the U having the finger stock inserted.

David
Oddly RF can still find a way in. A few years ago I was PM for an instrument currently on its way to Mercury (MIXS, one of 12 instruments on this https://sci.esa.int/web/bepicolombo . As part of the programme was two weeks testing at 3C test https://www.3ctest.co.uk/ . While an immunity scan was underway, the technician running that particular test piped up "who's using a mobile phone?". Even though a sealed, qualified door, and filtered feed through connections there was the tell tale spike well above the background. Sure enough one of my team was using her phone in the lobby outside the chamber.

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Old 2nd Jun 2020, 1:18 am   #18
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Default Re: Faraday chamber.

You can breach the screening of even a very good screened room in a number of ways.
You now set up currents in the shell that were not created by an incident wave. And the screened room itself becomes your radiator into the outside world. It upset the people who'd just spent an awful lot of money on two linked chambers.... one for the test subject, one for the test equipment.

So you have to be very careful about cable routing and diligent about keeping all the connections grouped close together where they pass through the skin (and connect their grounds/screens to the skin.

Ferrite beads on cables help a lot in realising the ultimate attenuation a chamber is capable of. For the measuring receivers etc, you soon see the difference between single screened cables (RG213 etc) versus double screened (RG223). You learn that N-types aren't bad IF the braid connection is well made (crimps seem better than compression types) and BNCs simply don't have solid enough ground connection, relying on springs a bit too much.

You learn a lot de-bugging a new screened room.

The biggest scare was coming in one morning and seeing the painter from the building maintenance department just about to start on it.... There were 'do not paint' stickers near the door. Maintenance boss said the colour of the chamber was unacceptable, it HAD to be painted. I said I couldn't argue with that, but give it a couple of months and the new chamber would be scrapped and replaced with a brand new one, because the existing one was about to be ruined... and the new one would be back to the chamber manufacturer's colour. He came round to seeing the point when someone mentioned cross-charging it to his department's budget.

So maintenance erected a partition wall all around the screened room complex, and painted that wall their approved colour!

Of course, it all lasted about 2 years before management decided to move departments around, and the screened rooms had to be moved downstairs

Having seen the price of a GTEM cell (sounds of Serge Gainsbourg and Jane Birkin in the distance ) I've wondered about the idea I came across of getting a lorry trailer type shipping container and welding up all the seams and doors, adding an RF proof door.

Maybe just because I'd enjoy the welding...

David
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Old 2nd Jun 2020, 9:02 am   #19
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Default Re: Faraday chamber.

Re : BNC and N types compression and crimp. Many don't make the compression joints properly.
There is rubber sleeve and an open ended metal top hat.

You put the nut and rubber sleeve on the jacket you strip the braid and fan it out evenly in a circle. You put the top hat between inner insulation and braid and slide it up inside as far as it will go. Now you slide down the rubber sleeve trapping the fan of braid against the top hat rim. Then trim back the wire fan to the edge of the rim and rubber. Now fit the centre pin and assemble.

The point here is that the nut applies force to the rubber which transmits it on to the fan of braid, then to the top hat and the body of the connector.

Many don't make the braid fan and then the only braid grounding is by "hopeful" swelling up of the rubber sleeve. You soon know you are up against one made like this when you can rotate the connector on the end of the cable and that is a test I make on every cable out of instinct. Not only have they no screening they are intermod generators.

I cut them off every time I find one.

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Old 2nd Jun 2020, 9:31 am   #20
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Default Re: Faraday chamber.

Many years ago I got to design the dimensions of a portable screened chamber to use at work on a receiver project. I thought it would be a good idea to make it such that it would just fit through a standard office door in its smallest dimension.

When it arrived there was much fuss and commotion because we couldn't get it beyond the goods in room. I had overlooked the fact that the big and chunky fasteners for the rear access plate and the front door hinges took it beyond the width of a standard door. Oh dear...

The solution for this was for me to find a suitable place to hide whilst the goods in team temporarily removed the door hinges and the rear plate. This was much harder than it sounds. Although we rarely use this chamber these days, I am convinced my mistake was the work of genius because we still have the chamber despite numerous clearouts of unused equipment. It is too much effort to take the hinges and plate off it such that it can be removed from the building.
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